Add accessibility_perftests

Recently there have been performance improvements in ui/accessibility.
This CL creates BUILD.gn entry for accessibility_perftests.exe which
will be used to run performance tests for this directory.

With this CL, I'm adding perf tests for the following methods :
AXPosition<>::AsTreePosition
AXPosition<>::AsLeafTextPosition

Bug: 928948
Change-Id: Iedbb274868a42009766de31d6f8b8731640847e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838264
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Adam Ettenberger <adettenb@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#708232}

diff --git a/BUILD.gn b/BUILD.gn
index a0153dd..93d9108a 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -147,6 +147,7 @@
       "//third_party/webrtc/rtc_tools:frame_analyzer",
       "//tools/perf/clear_system_cache",
       "//tools/polymer:polymer_tools_python_unittests",
+      "//ui/accessibility:accessibility_perftests",
       "//ui/accessibility:accessibility_unittests",
       "//ui/accessibility/extensions",
     ]